DUBLIN — A new Irish Republican Army faction in Northern Ireland claimed responsibility Monday for its first killing and defended the bloodshed as a necessary act of vengeance.

The group, a merger of factions that brands itself as simply the IRA, said in a statement to the Irish News in Belfast its members shot to death David Black this month because he worked as a guard at Northern Ireland’s top-security Maghaberry prison.

About 40 members of IRA factions are imprisoned there. The inmates have protested for more than a year against a policy of strip-searching them in search of weapons, drugs and cellphones. They have previously threatened to kill off-duty guards.

Black, 52, was shot as he drove to work on Nov. 1. He had worked as a guard for three decades and expected to retire soon.

He was the first prison officer killed in Northern Ireland since 1993, the year before the dominant anti-British paramilitary group, the Provisional IRA, began an open-ended truce that inspired Northern Ireland’s peace process. The Provisionals renounced violence and disarmed in 2005.

The group that claimed Black’s killing was formed in July by the merger of three anti-British splinter groups led by former Provisionals who still pursue violence in Northern Ireland. The merger represented an effort by breakaway IRA members to mount a more coherent campaign, given that most of their bombings and shootings fail because of faulty equipment or British intelligence tipoffs.

The new group’s statement said it had “a responsibility to protect and defend” its imprisoned members. It described the Black killing as a direct response to “the degradation” of prison strip-searching.

The three groups that now constitute the self-styled IRA faction are the Real IRA, Republican Action Against Drugs and Oglaigh na hEireann, the Gaelic equivalent of “IRA.”

The Real IRA was responsible for the deadliest bombing in Northern Ireland history: the 1998 car-bomb attack on the town of Omagh that killed 29, mostly women and children. The other factions are of more recent vintage, with the RAAD group focused on targeting criminal rivals in Northern Ireland’s second-largest city, Londonderry.

A fourth faction also still committed to violence, the Continuity IRA, has declined to merge.

Sinn Fein, the Irish nationalist party that delivered the Provisionals’ disbandment, said today’s IRA remnant had no coherent political strategy and should abandon violence too.

Gerry Kelly, a senior Sinn Fein politician and convicted Provisional IRA car bomber, said any fringe IRA factions “cannot deliver a united Ireland,” the traditional IRA goal of forcing Northern Ireland out of the United Kingdom and into the Republic of Ireland.

“These groups need to realize that they cannot derail the peace process and their actions will not resolve anything within the jail or in wider society. They are killing for the sake of killing and should stop immediately,” he said.

Black’s family barred Sinn Fein politicians from attending his funeral. Like most prison officers, he came from the province’s British Protestant majority and also was a member of the Orange Order, a conservative Protestant brotherhood that is despised by many Irish Catholics.